Our goal is to understand chronic blepharitis, the role of the meibomian gland in blepharitis and the effect of meibomian gland loss on the dynamics of ocular evaporation. Our hypothesis is that meibomian gland loss, which occurs in some forms of chronic blepharitis, leads to altered meibomian gland function, increased evaporation from the ocular surface, and increased tear osmolarity. By careful analysis of the data obtained on these blepharitis patients, we also hope to achieve a better under- standing of the characteristics of the blepharitis subsets and of the interrelationship between meibomian gland function (gland dropout, volume and viscosity) and the evaporative state (tear evaporation, osmolarity, and tear flow). To achieve these goals we will test our hypothesis by the following specific aims: 1) We will establish the range of normal meibomian gland function on the evaporative state of the eye for 150 patients over a rang in ages. We will measure five quantitative variables: meibomian gland dropout, evaporation rate, tear osmolarity, and tear flow by fluoropho- tometry, and two qualitative variables: excreta volume and viscosity. 2) We will establish the range of meibomian gland function in 150 patients with chronic blepharitis measuring the same variables and using the same age and sex distribution. 3) We will determine the correlation between evaporation rate and gland dropout, tear osmolarity, tear flow and excre- ta volume and viscosity in these 150 chronic blepharitis patients using the data collected above. 4) We will investigate the effect of induced meibomian gland dysfunction resulting from systemic Isotretinoin (accu- tane) on tear evaporation, osmolarity, and tear flow before, during and after treatment in 30 patients by measuring these same variables. 5) We will use our data to characterize better the subsets which comprise ble- pharitis by applying cluster analysis to the large data base collected from these 150 patients with blepharitis.